1. Field of the Invention
The present invention relates to a catalyst composition and method of using the same for the treatment of gas streams such as diesel engine exhaust streams by the oxidation of oxidizable components, such as hydrocarbons and carbon monoxide, reduction of nitrogen oxides of exhaust gases, and the reduction of volatile organic materials and organic particulates and other pollutants prior to being discharged to the atmosphere.
2. Description of Related Art
Diesel engine exhaust is a heterogeneous material which contains not only gaseous pollutants such as nitrogen oxides (NOx), carbon monoxide ("CO") and unburned hydrocarbons ("HC"), but also soot particles which comprise both a dry, solid carbonaceous fraction and a soluble organic fraction. The soluble organic fraction is sometimes referred to as a volatile organic fraction ("VOF"), which terminology will be used herein. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., VOF, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide and the oxidation catalyst catalyzes the SO.sub.2 to SO.sub.3 ("sulfates") with subsequent formation of condensable sulfur compounds, such as sulfuric acid, which condense upon, and thereby add to, the mass of particulates. The sulfates also react with activated alumina supports to form aluminum sulfates, which render activated alumina-containing catalysts inactive as disclosed in U.S. Pat. No. 4,171,288. Previous attempts to deal with the sulfation problem include the incorporation of large amounts of sulfate-resistant materials such as vanadium oxide into the support coating, or the use of sulfur-resistant support materials such as .alpha.-alumina (alpha), silica and titania.
The prior art also shows an awareness of the use of zeolites, including metal-doped zeolites (i.e. catalytically active), to treat diesel exhaust. For example, U.S. Pat. No. 4,929,581 discloses a filter for diesel exhaust, in which the exhaust is constrained to flow through the catalyst walls to filter the soot particles. A catalyst comprising a platinum group metal-doped zeolite is dispersed on the walls of the filter to catalyze oxidation of the soot to unplug the filter.
EPO 92/102161.4 discloses a catalyst for reducing the particle content and/or size in diesel engine exhaust by way of the zeolitic catalyst having acidic properties. The catalyst is stated to have properties enabling it to crack long-chain aromatic hydrocarbons. Zeolites include faujasite, pentasil and mordenite. Additionally, the faujasite and mordenite can be dealuminated. The zeolite is stated to contain one or several transition elements which can include copper, nickel, cobalt, iron, chromium, manganese and/or vanadium.
Japanese Application No. S63-95026 (Publication No. H1-266854, Oct. 24, 1989) discloses a catalyst for cleaning exhaust gas. The catalyst consists of zeolite, ion-exchanged with copper and carried on a fireproof carrier. The ion-exchange site is on the surface of the zeolite supercage and the coordination site of the oxygen atom for copper ion is a four-coordinate square.
Iwamoto, Catalytic Decomposition of Nitrogen Oxides, Petrotech 12, 888-893, 1989 is directed to the reduction of nitrogen oxides and various emissions from diesel exhaust. It is taught to use copper ion-exchanged ZSM-5 or mordenite or ferrierite. U.S. Pat. No. 4,934,142 discloses an exhaust emission control device comprising a first filter provided in an exhaust system of an engine to collect particulates contained in an exhaust gas. A second filter is provided downstream of the first filter to absorb an offensive odor component. The second filter is formed by an ion-exchange of copper ions of copper carried on a zeolite.
EPO Application No. 0 508 513 A1 discloses a method for treating diesel fuel engine exhaust to reduce emission of particulates having cores of carbonaceous material and condensable hydrocarbons deposited on the carbonaceous material. The condensable hydrocarbons in the exhaust are contacted with a catalytically active solid acid material having hydrogen ions releasably retained at acidic sites thereof. The solid acid material is desirably a Y-type zeolite with (H) cations (HY zeolite), or hydrolyzed multi-valent cations such as lanthanum (LaY zeolite), cerium (CeY zeolite) and calcium (CaY zeolite) and is supported on a ceramic or metal monolith.
WO 94/01926 entitled, "Improved Zeolite-Containing Oxidation Catalyst and Method of Use" discloses catalyst compositions for treating a diesel engine exhaust stream containing a volatile organic fraction. A catalyst composition comprises a refractory carrier on which is disposed a coating of a catalytic material comprising a catalytically effective amount of ceria having a BET surface area of at least about 10 m.sup.2 /g and a catalytically effective amount of a zeolite. It is also known to employ ceria and alumina as a support for a platinum group metal as a dual exhaust catalyst. The zeolite can be doped with a platinum group metal. In this composition the zeolite is employed to serve both to catalyze the oxidation of VOF and to crack the larger VOF molecules and, during period of relatively low temperature operation, to trap gas-phase hydrocarbons within the zeolite pores. If the zeolite has been doped with one or more catalytic metals or hydrogen, the trapped gas-phase hydrocarbons are brought into intimate contact with the catalytically active cations which facilitates oxidation of the hydrocarbons.
WO 97/00119 entitled, "IMPROVED DIESEL ENGINE EXHAUST GAS CATALYST AND METHOD OF USE" discloses a catalyst composition for treating a diesel exhaust stream comprising a catalytically effective amount of at least one platinum group metal on a support in the presence of at least one catalyst activity controlling compound which can be selected from the group consisting of compounds containing vanadium, gold, silver, iron and combinations thereof. The composition additionally includes a thermally stable ceria and a non-catalytic zeolite. The purpose of the zeolite is to adsorb and retain gaseous hydrocarbons below catalytic temperatures. The zeolite itself does not catalyze any of the components of the diesel exhaust stream. Zeolite is non catalytic and is not doped with catalytic materials such as platinum, iron and the like. A preferred zeolite is Beta zeolite.
WO 96/15992 entitled, "BASIC ZEOLITES AS HYDROCARBON TRAPS FOR DIESEL OXIDATION CATALYSTS" discloses compositions comprising at least one zeolite consisting essentially of a zeolite selected from the group of neutral zeolites and basic zeolites. The zeolite can be supported on a substrate carrier such as a honeycomb support which may be made of refractory metal material or metal material. The composition can further comprise oxidizing catalytic components such as at least one platinum group metal. The composition is particularly useful for treating gas streams comprising hydrocarbons. Gas streams of particular applicability include diesel engine exhaust streams which comprise hydrocarbons. Zeolites serve to adsorb the hydrocarbons at low temperatures. As the diesel exhaust gases heat up, the hydrocarbons release from the zeolite composition at a high release temperature range, typically above 175.degree. C. and more typically above 200.degree. C. The released hydrocarbons can then be oxidized.
WO 96/40419 is directed to a composition and method to treat diesel exhaust gas to remove carbon monoxide, hydrocarbons, and nitrogen oxides, and to minimize the production of sulfur trioxide, comprising contacting said gases under conversion conditions with an effective amount of a catalyst consisting essentially of a high activity noble metal supported on a two-component support that contains a major amount of at least one suitable zeolite and a minor amount of an inorganic refractory oxide and washcoated on a flow through monolithic carrier.
Other references which disclose process for removal of hydrocarbons carbon monoxide from gaseous compositions include U.S. Pat. Nos. 5,234,876, 5,248,643, 5,284,638, 5,292,991 and 5,244,852. Also of interest with regard to the use of zeolites in compositions to treat diesel exhaust gas are DE4226111A1 and DE4226112A2. References of interest also include Feeley, et al., ABATEMENT OF NOX FROM DIESEL ENGINES; STATUS AND TECHNICAL CHALLENGES, SAE #950747; M. Iwamoto, et al., JSAE (Japan) Review, 16, pp 21-25 (1995); and Burch, et al., AN INVESTIGATION OF THE MECHANISM OF THE SELECTIVE CATALYTIC REDUCTION OF NO ON VARIOUS METAL/ZSM-5 CATALYSTS: REACTION OF H2/NO MIXTURES" Catal. Lett., 27, pp 177-186 (1994).
The use of finely divided inorganic oxides containing vanadium and platinum group metal as active components is disclosed in U.S. Pat. No. 5,157,007. The catalyst is in the form of an open cell, monolith.
WO 94/22564 discloses a catalyst composition for treating diesel exhaust which includes ceria and optionally alumina as well as a Beta zeolite. A platinum group metal is employed to promote oxidation of CO and HC while limiting the conversion of SO.sub.2 to SO.sub.3.
As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation. This is because the engine exhaust is not at a temperature sufficiently high for the efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to employ high loads of the platinum group metal catalyst to increase catalytic activity at low temperatures. It is also known to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period until the exhaust reaches a more suitable, higher temperature. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature as disclosed, for example, in U.S. Pat. No. 5,125,231 in which platinum group metal-doped zeolites are employed as low temperature hydrocarbon adsorbents and oxidation catalysts.
Such efforts to improve upon the performance of diesel exhaust catalysts have been problematical. This is because, the low and high temperature operating conditions, the presence of SO.sub.2 and the need to effectively convert CO and HC to innocuous materials often impose competing requirements on diesel exhaust catalysts. For example, it is known that high loading of platinum group metals is required to convert CO and HC at low temperatures. However, high loading of the platinum group metal increases the rate of conversion of SO.sub.2 to SO.sub.3.
It is also known to modify the activity of the platinum group metals by adding appreciable amounts of vanadium oxide to the catalyst composition. Vanadium oxide reduces the activity of the platinum metal to thereby reduce the rate at which SO.sub.2 is converted to SO.sub.3. However, after a relatively short operating period, vanadium oxide begins to irreversibly deactivate the platinum group metal thereby decreasing the performance of the catalyst in the conversion of CO and HC.
It would therefore be a significant advance in the art of converting diesel exhaust to innocuous materials to provide a catalyst which effectively converts nitrogen oxides (NOx), CO and HC including the volatile organic fraction, while minimizing the conversion of SO.sub.2 to SO.sub.3.